It seems that whenever a triangle has been rotated, it becomes blurry on the edges. Triangle is drawn using illustrator
And if scaled down, it becomes worse on the edges. Any ideas / solutions to have a smoother edge?
The triangles in the top image are not blurry. They look fine to me. When you rotate an image made of pixels so that the edges are no longer horizontal or vertical, or rotate such a vector then rasterize it, or view such a vector image on a monitor (all of which have pixels), then obviously the pixels will be more visible - like they are stepped or jagged. This is unavoidable. Anti-aliasing usually takes care of this and helps smooth out the pixels. This might be what you are referring to as blurriness, but it's not! The alternative would be to have the image non-anti-aliased - the results would look terrible however. You'd end up with very jagged/noticeable pixels.
Example of triangles without anti-aliasing
The triangles in the image below your original look slightly over sharpened at the edges. When resizing in Photoshop it might be useful to use Bicubic (Smooth Gradients) option when resampling. This might be useful if you think the lines are being over sharpened.
Example resampled using Bicubic (Smooth Gradients).
Here's a zoomed in comparison of your reduced size image next to the one I resampled using the Bicubic (Smooth Gradients) option.